Helium stars as supernova progenitors

We follow the evolution of helium stars of initial mass $(2.2 - 2.5) M_\odot$, and show that they undergo off-center carbon burning, which leaves behind ${\mathbf \sim 0.01 M_\odot}$ of unburnt carbon in the inner part of the core. When the carbon-oxygen core grows to Chandrasekhar mass, the amount of left-over carbon is sufficient to ignite thermonuclear runaway. At the moment of explosion, the star will possess an envelope of several $0.1 M_{\odot}$, consisting of He, C, and possibly some H, perhaps producing a kind of peculiar SN. Based on the results of Waldman and Barkat (2007) for accreting white dwarfs, we expect to get thermonuclear runaway at a broad range of $\rho_c \approx (1 - 6) \times 10^9 \mathrm{g cm^{-3}}$, depending on the amount of residual carbon. We verified the feasibility of this scenario by showing that in a close binary system with initial masses $(8.5 + 7.7) M_{\odot}$ and initial period of 150 day the primary produces a helium remnant of $2.3 M_{\odot}$ that evolves further like the model we considered.